This invention relates to a ceramic sintered body having excellent strength and showing little change of dimensions produced by thermal treatment, and having a high melting point and a low thermal expansion. Especially, this invention relates to a ceramic sintered body for structural uses, e.g. as a portliner for an internal combustion engine.
Aluminum titanate is a specific ceramic having a high melting point and showing a low thermal expansion. Because of these characteristics, attempts have been made to utilize it practically. However, it is subject to the problem that its mechanical strength is low, since it is difficult to obtain a fine sintered body, and it is also subject to thermal instability in that it undergoes thermal cracking when heated up to 1250.degree. C. Also, thermal expansion and contraction of the sintered body are affected by the thermal treatment history, and its strength is very adversely affected by repeated heating and cooling.
This thermal treatment effect is the result of the aluminum titanate crystals having anisotropic thermal expansion characteristics, with the result that fine cracks are produced in the sintered body by a thermal treatment history of cooling after heating, thereby adversely affecting the strength of the sintered body. This formation of cracks on cooling occurs every time the ceramic sintered body is subjected to heating and cooling after sintering. It therefore causes problems of loss of strength and dimensional stability, particularly in cases where the sintered body is subjected to a cyclical heat treatment.
Various additives have been introduced in attempts to ameliorate these difficulties of aluminum titanate. For example, published Japanese Patent No. SHO.56-7996 discloses low thermal expansion ceramics containing 0.05 to 10.0 weight % of at least one of silicon and zirconium, calculated as SiO.sub.2 and ZrO.sub.2, with respect to the aluminum titanate. It is stated that by the introduction of prescribed contents of silicon and zirconium into this ceramic, grain growth is controlled, resulting in a ceramic which is thermally stable and which shows little strength deterioration after a history of thermal treatment.
The Journal of the Chemical Society of Japan (Nippon Kagaku Kaishi) (1981 No. 10) pages 1647 to 1655 reports the effects of various additives under the title "Effect of additives on the properties of aluminum titanate sintered bodies". This report describes the results of attempts to improve the cracking resistance in the low temperature region by blending in additives in order to suppress the growth of aluminum titanate crystals, while promoting sintering, increasing the mechanical strength and maintaining an apparent low thermal expansion. The additives investigated were Li.sub.2 O, B.sub.2 O.sub.3, SiO.sub.2, MgO, Cr.sub.2 O.sub.3, Fe.sub.2 O.sub.3, and ZrO.sub.2. The effect of these additives is examined and summarized. Whereas MgO, Fe.sub.2 O.sub.3 and ZrO.sub.2 have the effect of increasing the density of the sintered body, Cr.sub.2 O.sub.3 does not necessarily increase the density. Also, although Li.sub.2 O and B.sub.2 O.sub.3 showed an effect of promoting increased fineness, SiO.sub.2 showed a sintering promoting effect.
Furthermore, published Japanese Patent No. SHO.62-32155 discloses a composite with metal incorporating a ceramic material obtained by sintering a raw material whose chemical composition is: 50 to 60 weight % Al.sub.2 O.sub.3, 40 to 45 weight % TiO.sub.2, 2 to 5 weight % kaolin (corresponding to Al.sub.2 O.sub.3.2SiO.sub.2), and 0.1 to 1 weight % magnesium silicate, and having a particle size less than 0.6 micron. This patent document suggests that the thermal and mechanical properties of the aluminum titanate are improved by using a combination of aluminum silicate-containing and magnesium silicate-containing additives.
Although in such ceramic sintered bodies, various properties of the aluminum titanate are improved, for practical use there remains a need for aluminium titanate ceramics exhibiting even greater improvements in properties such as strength and thermal variation in length. For example, although it would appear that the combination of sintering adjuvants of the above-mentioned published Japanese Patent No. SHO.62-32155 is desirable, experience has shown that it is extremely difficult to obtain ceramic bodies exhibiting uniformly satisfactory strength and thermal expansion characteristics with the form of starting materials and additive contents set out in Japanese Patent No. SHO.62-32155.